A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
A photolithographic apparatus requires an illumination system that supplies a beam of radiation that is used to impart a pattern to the substrate. Of course, illumination systems for forming beams may also be used in applications other than photolithography where a beam or radiation is needed. An embodiment of such an illumination system contains mirrors to reflect the beam. In particular when an EUV beam is used mirrors are preferred over lenses. Preferably metal mirrors are used. Metal mirrors may be used that comprise a thick metal layer to reflect all of the radiation at a grazing angle of incidence, or alternatively mirrors may be used, comprising a stack alternating layers of metal and layers of non-metal. Metal mirrors with a thick metal layer are mostly used with a grazing incidence beam. Metal mirrors with a stacked structure are also used under other angles of incidence, even under (near) normal incidence conditions. Typical metals used in such mirrors are Molybdenum and Ruthenium. In the stacked structures the layers between metal layers are often made of silicon.
However, the use of such mirrors has its problems. Molybdenum and Silicon are susceptible to oxidation when exposed at the surface. Therefore mirrors containing layers of these materials are often provided with a top layer, which protects against oxidation (for example ruthenium may be used for this).
Also the mirrors tend to get contaminated during use. From US patent application publication No 20060278833, assigned to the same assignee, it is known that use of a gas containing hydrogen (in particular hydrogen radicals) can be used to remove contamination from an optically reflective surface in a EUV lithographic apparatus. It has been found that Sn and C contamination can be effectively removed from reflective surfaces in this way.
Unfortunately atomic hydrogen can also cause cracks in the reflective surface when the mirror has a layer of certain metals, such as an exposed ruthenium or molybdenum layer, that can be reached by the hydrogen radical. This effect has been described in a US patent application assigned to the assignee of the present application (assignee docket P 2263.000). In this patent application it was described that a coating of Si3N4 can be used to protect against cracking. However, such a coating detracts from reflection and may be difficult to deposit for certain mirror configurations
An EUV mirror containing a stack of layers of alternately Si and MoSi2 is known from an article titled “High-temperature MoSi2/Si and Mo/C/Si/C multilayer mirrors High-temperature MoSi2/Si and Mo/C/Si/C multilayer mirrors”, published in a poster session of the 3rd International EUVL Symposium 01-04 November 2004 Miyazaki, Japan. This reference is limited to multilayer mirrors and it does not mention hydrogen cleaning. Moreover a MoSi2/Si mirror has a relatively low peak reflection (approx. 40%) at 1.5 degrees to the perpendicular.